There are several known color compensation techniques for color image signals, for obtaining brighter colors upon display. One example of such a technique can be found in Japanese Laid-Open Patent Application Tokukaihei 03-266586/1991 (published on Nov. 27, 1991, hereinafter referred to as Document 1). In this example, color compensation is performed by using six color components of a signal: the three primary colors, R (Red), G (Green), and B (Blue); with Y (Yellow), M (Magenta), and C (Cyan) as complementary colors of those primary colors.
The color compensation of Document 1 is carried out as follows. For the RGB image signal components of a signal, the components of the three primary colors and the components of the three complementary colors are individually extracted. Then, the component of each color is multiplied by an adjustment coefficient which is determined differently for each color in advance. Further, the calculated value for color compensation is added to the original RGB signals so that new corrected color signals R′G′B′are created.
For example, a color image signal in which the respective signals of R, G, B are contained by a ratio of 0.8:1.0:0.2, respectively, is expressed as 0.8R+1.0G+0.2B. This expression can be modified as 0.2 (R+G+B)+0.6 (R+G)+0.2G. After the modification, the original signal is divided into three components: (R+G+B), (R+G) and G. Here, (R+G+B) denotes a white component and (R+G) denotes an Y component.
Since the white component is not used for calculation, the original signal is divided into an Y component and a G component. The Y component and the G component are then respectively multiplied by predetermined constants, and the respective calculation results are then added to the original RGB signals. Thereafter, the R′G′B′signal having been through color compensation, is outputted.
With reference to FIGS. 13 and 14, the following will describe a change of gradation level of a signal through the color compensation with the foregoing technology of Document 1.
FIG. 13 shows a so-called HSL color model, which indicates distributions of luminance and saturation of the color. FIG. 13(a) is a perspective view of the HSL (a color model expressed by Hue, Saturation and Luminance), while FIG. 13(b) shows a circle as an upper view of the inverted-cone-shaped HSL, and a triangle as a cross-sectional view taken along a line between a point of Y (Yellow) 1303 and a point of B (Blue) 1304. The closer to the circumference, the greater the saturation (the greater the gradation level denoting saturation), and the more upward (the center of circle 1302 is white) from the top 1301 (black) of the cone, the greater the luminance (gradation level denoting luminance).
FIG. 14 is a schematic view showing a change in gradation level of luminance and saturation of the Y component and the B component through the color compensation with the foregoing technology of Document 1. FIG. 14(b) shows the Y component of the input color image signal with enhanced gradation level. As shown in the Figure, in the color image signal having been through color compensation, colors are properly modified for the domain close to the center (the center denotes an achromatic color, and the color becomes more mixed from outside to inside the circle) of HSL. However, the color in the vicinity (the circumference denotes a monochromatic color, and the color becomes more monochromatic from inside to outside the circle) of the circumference of HSL may fall outside the circumference (see 1401 in FIG. 14) of the circle. For example, assuming that the maximum saturation is 255 in gradation level, and the Y component after separated from the input signal is multiplied by a constant, the obtained value may exceed the gradation level of 255. Thus, the color image signal with the color outside the range will fail to properly display an image.
As described, since the color compensation according to Document 1 is performed by calculation in which monochromatic colors and mixed colors are corrected together, it fails to obtain a desired image, or fails to create and display an image with higher quality.
More specifically, through color compensation with the foregoing technology, the calculated value may become higher than the upper limit of saturation or luminance in one or some color components. As such, color compensation fails in domains of monochromatic colors or domains close to monochromatic colors. By having the color with a color component improperly corrected, the displayed image contains both properly modified pixels and improperly modified pixels. This thus results in the displayed image becoming partially unnatural.
Further, in color compensation with the foregoing technique, the white component is not used for color conversion calculation after extracted from the input signal. Therefore, there will be an only small difference in saturation or luminance between monochromatic colors and mixed colors. Thus, monochromatic colors fail to be enhanced to generate a bright image.